The invention relates to a polarizing structure comprising a glyoxal based adhesive system. It also relates to a polarized ophthalmic lens comprising such polarizing structure.
The optical or transmission properties of ophthalmic lenses can be modified by adding a film onto a lens surface. These films are often referred to as functional films such as polarizing films which are capable of reducing glare while providing a high level of optical transmission.
There are many ways to make polarized lenses, such as by injecting a lens against a polarizing film or structure, casting a lens adjacent a polarizing film or structure, casting a lens containing a polarizing film or structure, or laminating the film or structure with a glue. It is known to manufacture a polarized optical lens by laminating a polarizing structure onto a lens surface. Typically, an adhesive layer, for example thermal or UV curable glue, holt melt adhesives (HMA) or pressure sensitive adhesive (PSA) is placed between one surface of the optical lens and the polarizing structure, so as to permanently retain the structure on the surface of the optical lens. Such polarizing structure is obtained by incorporating a polarizing film, for example a polyvinyl alcohol (PVA) polarizer between two protective films. Materials for the protective layers can typically include cellulose triacetate (TAC), polycarbonate (PC), cellulose acetate butyrate (CAB), poly(methyl methacrylate) PMMA, poly(ethylene terephthalate) (PET) or polyamide (PA). The polarizing structure is laminated together with a water based adhesive between the protective films and the PVA film, such as a polyvinyl alcohol (PVOH) based adhesive. This adhesive can be sensitive to water; therefore, for example the TAC/PVA/TAC structure has very poor wet adhesion and can be easily separated when contacted with water.
Because of the water sensitivity of this polarizing structure and its poor wet adhesion, the polarized lens comprising such structure has delamination problems during wet wheel edging during processing of polarized lenses. More particularly, it has been seen that the TAC/PVA/TAC polarizing structure is separated at the edge of the lens due to the combined effects of wheel edging force with water during the edging process.
What's more, the dry peel force of said polarizing structure, i.e. the measurement of the force applied in order to peel off one of the films of said structure, is an important parameter to observe. Indeed, delamination problems are likely to be observed during dry edging or after repeated use of an ophthalmic lens comprising such a polarizing structure.
The adhesive system used in the polarizing structure is thus the key element in order to obtain a structure which maintains its integrity and does not delaminate.
According to the prior art, glyoxal is widely used as a film additive, for example to improve the properties of polyvinyl alcohol (PVA) films. In the Polymer Journal, Vol 25, No. 12, pp 1295-1302 (1993) article entitled Effects of the Degree of Cross Linking on Properties of PVA Membranes, the authors propose using glyoxal and glutaraldehyde as film additives.
U.S. Pat. No. 5,114,999 discloses using glyoxal as a binder insolubilizer in a paper coating composition.
Glyoxal is also used as a crosslinking agent in adhesive formulations, for example, as a crosslinking agent for polyvinyl alcohol (PVOH) adhesive used in PVA and cellulose triacetate (TAC) film lamination. Glyoxal is a well known crosslinking agent within the PVA film industry to improve water resistance.
However, there has not been a suggestion to use it as an adhesive to bond PVA polarizing film and TAC film. Further more, in the prior art like U.S. Pat. No. 7,704,572, glyoxal is always used as an additive, for example as that is a cross-linking agent used in a minor quantity in an adhesive where the adhesive which represents the main compound component of the formulation. Other approaches suggest using metal crosslinking materials (U.S. Published Patent Application 2008/0278810) and other crosslinking agents such as glyoxal (U.S. Published Patent Application 2006/0155061) in PVOH adhesive to improve the film's water resistance. However, tests using these proposed adhesives failed to achieve the minimum requirements for adhesion. Therefore an adhesion problem arises that needs a solution.
In all of the prior approaches, glyoxal was used as a film additive or crosslinking agent and not as the main adhesive component.